1. Technical Field of the Invention
This invention relates to a method of purifying aromatic dicarboxylic acids, more particularly, to a method of purifying and recovering the thus highly purified aromatic dicarboxylic acids via their organic amine salts.
2. Description of the Related Art
From the standpoint of reuse of resources and protection of environment, a variety of resin products are in fact practically recovered and reused, or intensive investigations are conducted for recovery and reuse of resin products.
Certain kinds of polyester resins contains aromatic dicarboxylic acids as structural units. For example, an aromatic dicarboxylic acid such as terephthalic acid is contained in polyethylene terephthalate resins or polybutylene terephthalate resins, whereas naphthalene dicarboxylic acid is contained in polyethylene naphthalene-dicarboxylate resins. Such aromatic dicarboxylic acids are practically recovered together with alkylene glycols by hydrolysis of such polyester resins. However, since molded articles of polyester resins usually contain colorants, when such articles are hydrolyzed, the recovered aromatic dicarboxylic acids are usually colored. Hence, it is necessary that the recovered aromatic dicarboxylic acids be purified so that they can be reused as materials for production of polyester resins.
Terephthalic acid, one of the most widely used aromatic dicarboxylic acids as a material for production of polyester resins, has a low vapor pressure even at a high temperature, so that it is ritually impossible to purify the acid by distillation. Moreover, since the acid is substantially insoluble in many organic solvents, it is also impossible to purify the acid by recrystallization.
However, in general, an aromatic dicarboxylic acid, inclusive of terephthalic acid, forms water soluble salts. Thus, the aromatic dicarboxylic acid can be purified with use of usual purifying means. For example, an aqueous solution containing such a water soluble salt of the acid is prepared and then the solution is decolorized by the use of an adsorbent, such as activated carbon, or the salt is recrystallized from the solution, thereby to purifying the aromatic dicarboxylic acid.
As disclosed in Japanese Patent Publication No. 44-20616, there is already known a method to purify crude terephthalic acid wherein the crude terephthalic acid is reacted with ammonia to form an aqueous solution of diammonium terephthalate, and the solution is decolorized, and is then heated to a temperature of 90.degree.-95.degree. C. so that the salt decomposes and crystallizes as monoammonium terephthalate. The salt is collected by filtration and decomposed at a temperature as high as 200.degree.-210.degree. C. and thus the liberated terephthalic acid having a high purity is recovered. However, the method needs such a high temperature as abovementioned to decompose the monoammonium terephthalate, and hence the method is not readily put to practical industrial operation.
A further method is also known, as disclosed in Japanese Patent application Laid-open No. 46-1369, wherein an aqueous solution of diammonium salt of crude terephthalic acid or isophthalic acid is reduced, and the solution is heated to convert the diammonium salt to a monoammonium salt. The salt is recovered as a highly purified precipitate, and the precipitate is then heated to liberate terephthalic acid or isophthalic acid having a high purity, followed by recovery of the acid. The method also needs a high temperature, as high as more than 200.degree. C., to decompose the monoammonium salt to liberate terephthalic acid or isophthalic acid. Thus, the method is also not readily put to practical industrial operation.
As set forth above, the known method comprises preparing an aqueous solution of ammonium salt of a crude aromatic dicarboxylic acid, purifying the solution, thermally decomposing the ammonium salt to liberate the aromatic dicarboxylic acid having high purity, and then recovering the acid, and thus has a large number of problems, as above mentioned. In addition, since the ammonium salt of an aromatic dicarboxylic acid does not have a high solubility in water, and a concentrated solution can not be prepared, the method is not efficient as a further problem, an acid amide maybe by-produced when the ammonium salt is decomposed and the acid amide contaminates the recovered aromatic dicarboxylic acid.